The particle size distribution of radioactive aerosols, which originated from the released radioactive materials from the Fukushima Daiichi nuclear power plant accident, has been observed using the Andersen-type classifier combined with a high volume air sampler. Estimated activity median aerodynamic diameters (AMADs) of 131 I-, 134 Cs-and 137 Cs-aerosols were ranging from 0.56 to 0.60 lm, which were larger than that of 7 Be-aerosols existing as an aerosol including cosmogenic radionuclide, about 0.2 lm.